**3.3. Three-tier architecture**

Three-tier architecture is most commonly used to build web applications. In this model, the browser acts like a client, middleware or an application server contains the business logic, and database servers handle data functions. This approach separates business logic from display and data [10]. So the three layers commonly known as: presentation layer (PL/UI), business logic layer (BLL) and data access layer (DAL) as shown in **Figure 4**.


## **3.4. N-tier architecture**

Terms layer and tier are often used interchangeably but one point of difference is that a layer is a logical structuring mechanism for the elements that make up the software solution. That means logical software component groups, mainly by functionality, are used for software development purpose. By contrast, a tier is a physical structuring mechanism for the system infrastructure [11].

**Figure 4.** Three-tier architecture.

from the data layers. However, it does not divide application layers so they can be utilized separately. This makes them difficult to update and not specialized. The entire application

Three-tier architecture is most commonly used to build web applications. In this model, the browser acts like a client, middleware or an application server contains the business logic, and database servers handle data functions. This approach separates business logic from display and data [10]. So the three layers commonly known as: presentation layer (PL/UI), business

**a.** Presentation tier (Level 1): This provides the application's user interface (UI). Being the topmost level it displays information related to user oriented functionality responsible for managing user interaction with the system. This acts as common bridge into core business

**b.** Business logic tier (Level 2): This is also called application layer as it controls an application's functionality by performing detailed processing. This layer implements the core functionality of the system encapsulating the relevant business logic. It has components

**c.** Data access tier (Level 3): This includes data persistence mechanisms like database servers, file shares, etc. providing access to data hosted within system and data exposed by other networked systems. The data layer exposes generic interfaces that can be consumed by components in the business layer. It also provides an API to application layer that exposes methods of managing the stored data without out casting dependencies on the

Terms layer and tier are often used interchangeably but one point of difference is that a layer is a logical structuring mechanism for the elements that make up the software solution. That means logical software component groups, mainly by functionality, are used for software development purpose. By contrast, a tier is a physical structuring mechanism for the system infrastructure [11].

must be updated because layers are not separated.

logic encapsulated in business layer.

data storage mechanisms.

**3.4. N-tier architecture**

exposing service interfaces for callers to use.

logic layer (BLL) and data access layer (DAL) as shown in **Figure 4**.

**3.3. Three-tier architecture**

**Figure 3.** Two-tier architecture.

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Like an individual running server is one tier and several running servers may also be counted as one tier. Layer software implementation has many advantages and is a good way to achieve N-tier architecture. Layer and tier may or may not exactly match each other. Each layer may run in an individual tier. However, multiple layers may also be able to run in one tier.

N-tier implies more than three levels or tiers involved as depicted in **Figure 5**; mostly additional tiers are associated with business logic tier. Some layers in 3-tier can be broken further into more layers. These broken layers may be able to run in more tiers. For example, application layer can be broken into business layer, persistence layer or more. Presentation layer can be broken into client layer and client presenter layer [12]. So, in order to claim a complete N-tier architecture, client presenter layer, business layer and data layer should be able to run in three separate computers (tiers).


**Figure 5.** N-tier architecture.
